1. Field of the Invention
The invention relates to computer communications and more particularly to communications with a limited-access computer for controlling critical functions.
2. Background Art
A number of computers are used to control critical functions, such as controlling an airplane in flight, and access to data in such computers must necessarily be controlled since an unauthorized change in data could have disastrous effects. The same computer which controls critical functions may also be used for more routine functions such as storing noncritical data pertaining to the operating environment. In the context of in-flight computers, referred to as flight management computers, these have become an important part in the operation of all modern airplanes. The flight management computer reduces pilot workload by relieving the pilot of more mundane management tasks and automatically controls the airplane in four dimensions from just after takeoff to just before landing. Before the flight begins, the flight management computer must be initialized by the entry of such information as the intended route, the selected cruise altitude, and airplane gross weight. Before or during a flight, the pilot may enter additional data to fine tune the selected profile. Data such as winds aloft, although not required, enable the computer to more accurately predict estimated times of arrival. During the flight, the pilot may be required to report the expected time of arrival as well as changes in the expected time of arrival due to delays, since such information is of great importance to airlines operating the aircraft or air traffic control. The computer, and its programs and data structure, are typically certified by an appropriate regulatory agency before being approved for use as an in-flight control computer.
Prior art arrangements are known for communicating with on-board flight management computers via a ground computer and a standard air-to-ground data connection. These communications include messages from the ground computer to the on-board computer, referred to as uplinks, and messages from the on-board computer to the ground computer, referred to as downlinks. Typically, these messages have included a so-called imbedded message identifier (IMI) used to designate the type of data in the message. Another designator referred to as an imbedded element identifier (IEI) identifies the data elements within each message. Inside the on-board computer are a number of processing routines, each of which was dedicated to processing a separate IMI/IEI. In prior art systems, these routines are typically hard coded in the on-board computer and any change in the message format, such as the addition of an IMI/IEI combination requires modification of the on-board computer and recertification by the appropriate regulatory body. Such modification and recertification is both expensive and time consuming.
In the prior art system the on-board flight management computer was typically programmed to periodically transmit information to the ground computer in the form of a downlink message. Such a downlink message is initiated on the basis of the occurrence of a predefined trigger condition or in response to a pilot selection from a predefined prompt display. In the latter case, the flight management computer, in response to predetermined trigger conditions, provides a prompted display to the pilot and upon an indication that a particular item has been selected, the on-board computer transmits the selected information to the ground computer. In the prior art system, it is not possible to change the trigger conditions which control the prompt display or the direct trigger response without modification of the hard coded area of the on-board computer and recertification.